(a) Field of the Invention
The present invention relates to a backlight system for display devices including liquid crystal displays, or for advertising means, or for lighting devices and, more particularly, to a backlight system which can serve to produce high luminance in display device applications.
(b) Description of the Related Art
Generally, backlight systems include a light-guide panel, one or more light sources provided at one or more edges of the light-guide panel with one or more light reflecting lamp housings, a light reflecting film positioned under the light-guide panel, and so on. The ray profile just emerging from the light-guide panel is inappropriate for being directly applied to the display devices. So, it is necessary to modify the ray profile. For this reason, a plurality of optical films should be employed with the light-guide panel. However, when a large number of optical films are employed for that purpose, it involves a complicated configuration and increased production cost. FIG. 10 is an exploded perspective view of a liquid crystal display using a backlight system based on U.S. Pat. No. 4,542,449 where a relatively large number of optical films are introduced. As shown in FIG. 10, the backlight system includes a light source 1, a light reflecting lamp housing 2, a light-guide panel 3, and a light reflecting film 4. The light source 1 with a light reflecting lamp housing 2 is provided at an edge of the light-guide panel 3, and the light-guide panel 3 is sequentially overlaid with a light diffusing film 5, a first and a second light collimating films 6 and 7, and finally a liquid crystal panel 8. A predetermined pattern (not shown) is formed at a top or bottom surface of the light-guide panel 3. A lenticular layer 9 having a plurality of prisms each with a right-angled isosceles triangular shape is formed at top surfaces of the first and of the second light collimating films 6 and 7, respectively. The light collimating films 6, 7 are placed one over the other such that the longitudinal direction of each lenticular layer on the films 6, 7 is at a predetermined angle 90xc2x0 to one another. Since a number of optical films are used, it can be said that such a configuration involves complicated fabrication steps, and also, increased production cost.
In order to enhance the overall luminance of display devices using the backlight system as shown in FIG. 10, a reflective polarizer (not shown) may be additionally employed over the second light collimating film 7. When the liquid crystal panel 8 is placed over the reflective polarizer, an absorptive polarizer (not shown) can be additionally interposed between them. Cholesteric liquid crystal (CLC) polarizers or DBEF(copyright) of Minnesota Mining and Manufacturing Company can be used as the reflective polarizer. The CLC polarizers have optical characteristics of reflecting one of two circularly polarized components of the incident light while transmitting the other circularly polarized component of the incident light. In accordance with the optical characteristics of the liquid crystal panels used, the circularly polarized component of the light emerging from the CLC polarizer needs to be changed into a linearly polarized component by using a xcex/4 retarder. In contrast, the DBEF(copyright) has optical characteristics of reflecting one of two linearly polarized components of the incident light while transmitting the other linearly polarized component of the incident light. When such a reflective polarizer is employed in the backlight system, it is theoretically possible to obtain an 100% enhanced luminance compared to that of the backlight system having no reflective polarizer if any light absorption does not occur inside the backlight system. However, in the conventional backlight system with a reflective polarizer, the actually achieved enhancement rate of the luminance is much lower than the expected value. For example, in portable computers using a backlight system with two light collimating films as shown in FIG. 10, low luminance enhancement rates of from 25% to 30% are exhibited. This is because such a backlight system has a complicated configuration, hence a considerble amount of light is lost while recycling through the various optical components in the backlight system.
It is an objective of the present invention to provide a backlight system which can be cost-effectively fabricated with a simplified structure using a reduced number of optical films.
It is another objective of the present invention to provide a backlight system which can greatly enhance luminance when used with a reflective polarizer.
These and other objectives can be achieved by a backlight system according to the present invention, which includes one or more light sources, a light-it-guide panel, a light reflecting film, an anisotropic light diffusing film (ADF), and a light collimating film. One or more light sources with one or more light reflecting lamp housings are positioned at one or more edges of the light-guide panel. The light-guide panel is made of transparent dielectric materials. The light reflecting film is placed below the light-guide panel. The ADF is placed over the light-guide panel and is made of transparent dielectric materials. The ADF has directionally different diffusing properties. The light collimating film is placed over the ADF and is made of transparent dielectric materials. One surface of the light collimating film is smooth and faces the ADF. The other surface of the light collimating film includes a lenticular layer having a plurality of prisms and extending in a predetermined direction. The cross-section of the lenticular layer is of a series of triangles. Additionally, the reflective polarizer can be placed over the light collimating film.
One surface of the light-guide panel, facing the ADF includes a lenticular layer extending in a predetermined direction, and the other surface of the light-guide panel, facing the light reflecting film includes a predetermined pattern. The local density of patterns becomes larger at positions more distant to the light source than those closer to the light source 21. The lenticular layer has a plurality of prisms and thus the cross-section of the lenticular layer is a series of triangles, or particularly, a series of isosceles triangles. Each prism has a peak and a peak angle "xgr". The peak angle "xgr" of the prism ranges from 70xc2x0 to 110xc2x0 and the peak angle "xgr" of 90xc2x0 is preferred in particular. The peak of the prism is in a distance of 100 xcexcm or less from the peak of the adjacent prism. The longitudinal direction of the lenticular layer of the light-guide panel is in an angular relationship of from 70xc2x0 to 110xc2x0 with respect to the positioning direction of the light source.
The ADF has a major axis arid a minor axis. The major and the minor axes of the ADF correspond to the long arid the short axes of a screen image formed by the light rays emerging from the ADF, respectively. The direction of the major axis of the ADF is in an angular relationship of from 70xc2x0 to 100xc2x0 with respect to the positioning direction of the light source. The direction of the minor axis of the ADF is in an angular relationship of from xe2x88x9220xc2x0 to +20xc2x0 with respect to the positioning direction of the light source. In particular, it is preferred that the ADF is placed such that the major and the minor axes of the ADF are perpendicular and parallel to the positioning direction of the light source, respectively. The ADF has an anisotropy ratio xcexa9/xcfx89 of 2 or more, where xcexa9 is an angle indicating the full width half a maximum (FWHM) of the intensity of the diffused light along the major axis and xcfx89 is an angle indicating FWHM of the intensity of the diffused light along the minor axis when collimated light is normally irradiated to the ADF. The angle xcexa9 is preferred to be 30xc2x0 or more. As an ADF, a holographic light diffusing film fabricated by holography is preferred.
The longitudinal direction of the lenticular layer included in the surface of the light collimating film is in an angular relation of from xe2x88x9220xc2x0 to +20xc2x0 to the positioning direction of the light source. In particular, it is preferred that the longitudinal direction is parallel to the positioning direction of the light source. The peak of each prism of the lenticular layer included on the one surface of the light collimating film is in a distance of 70 xcexcm or less from the peak of the adjacent prism. The angle a between a first inclined side of the prism being farther from the light source and the smooth surface of the light collimating film is if the range of from 40xc2x0 to 90xc2x0, while the angle xcex2 between a second inclined side of the prism being closer to the light source and the smooth surface of the light collimating film is in the range of from 40xc2x0 to 60xc2x0.